Gypsum is the calcium sulfate dihydrate [DH] of the formula CaSO4.2H2O. Gypsum deposits exist around the world and have been used for centuries primarily in the building industry for structural and decorative purposes. More recently synthetic gypsum has come available as a byproduct from chemical processes or from the scrubbing of sulfur dioxide from the flue gases of coal burning power stations. The main commercial value from the use of gypsum results from its ability to lose three quarters of the water combined in the gypsum crystal upon heating, a process called calcining as illustrated in the reaction shown below.CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O
[gypsum] [calcium sulfate hemihydrate or plaster of Paris]
Upon further heating at higher temperatures the hemihydrate will lose the remaining water and form soluble anhydrite or anhydrite III [AIII], which has a similar crystallographic structure to hemihydrate and is easily reconverted to hemihydrate by absorption of water vapor from the atmosphere. Exposure of hemihydrate to high humidity will not only convert any soluble anhydrite to hemihydrate, but will also slowly convert hemihydrate to gypsum and reduce the overall reactivity of the hemihydrate plaster, a process commonly called aging a plaster. Even further heating will result in the hemihydrate or soluble anhydrite converting to the insoluble anhydrite form, Anhydrite II [AII].
When the hemihydrate [HH] form is mixed with water to form a slurry at room temperatures, the hemihydrate dissolves in water and recrystallizes as gypsum, solidifying in the process. At room temperature hemihydrate is more soluble in water than gypsum, causing the hemihydrate to dissolve and the gypsum to precipitate. On a pure basis [100% pure gypsum] only 18.6 ml of water is required to convert 100 g of hemihydrate to gypsum.
There have been several methods demonstrated for the dehydration or calcination of gypsum to plaster of Paris, and various different types of hemihydrate produced by these different processes. The most commonly produced calcium sulfate hemihydrate is the “beta” form, in which the gypsum is finely ground and then calcined at high temperatures under normal atmospheric conditions to give a fast setting hemihydrate material. Another common type is called “alpha” in which the dehydration process is carried out under pressure conditions greater than atmospheric. One of the major differences between the alpha and beta forms of hemihydrate is the amount of water that is required to be mixed with the powdered hemihydrate to give a pourable slurry (i.e. the water demand). After vigorous mixing of the hemihydrate with water, a typical beta hemihydrate plaster will require between 75-100 ml of water per 100 g of plaster to give a pourable slurry. A typical alpha hemihydrate, on the other hand will require only 28 to 45 ml of water to give a pourable slurry with 100 g of plaster. There has been much discussion in the scientific literature regarding the differences between alpha and beta hemihydrate, and indeed between hemihydrate and forms with 0.67 or 0.8 moles of water per CaSO4. For all practical purposes however, it would appear that all of these forms have essentially the same crystal structure. The difference between the hemihydrate and the more hydrated forms appears to be more water molecules being found in the open channels parallel to the crystallographic C axis. Although there appears to be some minor differences between the powder diffraction patterns of alpha and beta hemihydrate, the most recent thinking is that the beta hemihydrate is simply a more stressed and disordered form of the alpha hemihydrate bassanite structure.
Without being limited by theory, it is believed that this difference in water demand between alpha and beta hemihydrate is caused by a combination of physical and chemical effects resulting from the calcination process used to manufacture the hemihydrate. The beta hemihydrate calcination results in a stressed and disordered hemihydrate particle which will break into finer particles upon mixing in water. The interior surfaces of these fine particles are often highly charged resulting in a structured double ion layer surrounding these particles when mixed in water. The alpha hemihydrate particles, however, even when finely ground do not disintegrate into these fine particles and are generally lower in surface energy resulting in a less water being required to make a pourable mix, even after exposure to high shear forces. The rheological properties of aqueous hemihydrate mixtures are dependent on the surface chemistry and the particle size and shape of the hemihydrate particles after mixing in water.
The beta plasters are used in applications where a light weight fast setting product is required, whereas the alpha product is used where it is more important to have high strength and/or excellent detail in the casting of the setting plaster.
Whether alpha or beta hemihydrate is used, more water than is chemically required for hydration is added to the powder to achieve a pourable slurry. In most cases this extra water must be removed by a drying process which is very energy intensive and expensive. As a result there is an advantage to use a low water demand plaster in these cases to save drying costs. This is especially true when beta hemihydrate plaster is used, since much more water is mixed with the plaster than is needed to hydrate the hemihydrate to gypsum. A typical ½ inch (12.5 mm) thick gypsum board made with fast setting beta plaster, for example, needs to dry about 3.6 to 4 kg of water from each square meter of board whereas if it could be made with alpha hemihydrate then only about half of this amount of water would need to be dried off. The low water demand alpha plasters, however, have different setting properties making them unusable for some applications. These setting properties of an alpha hemihydrate are much too slow to be commercially viable for a modern gypsum board line.
Efforts have been made to reduce the evaporative load of these dryers by using chemicals such as dispersing agents [naphthalene sulfonates [NS], lignin sulfonates, melamine resins, etc.] to modify the surface properties of the hemihydrate particles in suspension and thus reduce the amount of water needed to make a pourable mix. These chemicals are quite expensive and limited in their effectiveness such that the water demand can be practically reduced by no more than 15% in most cases. These compounds are also often called water reducing agents or superplasticizers in the gypsum and cement industries.
There are two common commercial methods to make the low water demand alpha plaster, a “dry” process wherein lump gypsum rocks are calcined at high temperatures and pressures by live steam in a closed vessel and a “wet” process wherein the gypsum is slurried in water and calcined at high temperatures and pressures in a slurry to give the hemihydrate that needs to be filtered and dried before use. Note that the starting material for both the wet and dry processes is gypsum, in the former case in lump form and the latter case as a finely divided gypsum powder suspended in water in the autoclave.
There are also several different techniques to make the beta plaster, examples being a simple open tray in an oven, a rotary kiln, a commonly used kettle process operating in either a batch or continuous mode, as exemplified in FIG. 2, or flash calcined techniques where the gypsum is exposed to high temperature gases for a short period of time to remove the combined crystal water in the gypsum. The plaster characteristics resulting from these various processes can be quite different from one another not only as a result of the calcination equipment used but also the process parameters implemented during calcination. In general, however all of these processes under all conditions result in a hemihydrate plaster of water demand higher than those found for the alpha hemihydrate processes.
The ideal calcination to produce either the alpha or beta plasters will result in complete conversion from gypsum to hemihydrate. In practice, however, other species are produced: residual uncalcined gypsum, soluble anhydrite, insoluble anhydrite, or perhaps even calcium oxide.
It is well known in the industry that if the plaster is overcalcined so that some insoluble anhydrite is produced then the pourable water demand of the resultant plaster can be reduced. This is because some of the gypsum has been converted to the inert anhydrite form and is no longer available to set, as well as behaving as a surface treatment to the hemihydrate preventing it from disintegrating upon mixing. This practice has the disadvantage of restricting the setting characteristics of the resulting slurry and reducing the strength development properties of the setting slurry.
Similarly, different processes have been described where treatments are applied to the beta plaster to reduce the water demand in a manner similar to the natural aging process described earlier. U.S. Pat. No. 3,898,316 to Flood describes an aridization process whereby soluble salts are added to a continuous calcination to reduce the water demand. U.S. Pat. No. 3,415,910 to Kinkade describes a two step process whereby the gypsum is calcined to hemihydrate and then re-wetted and heated in the kettle once again to give a low water demand plaster. U.S. Pat. No. 4,533,528 to Zaskalicky describes the continuous calcination of wet synthetic or chemical byproduct gypsum to give a beta plaster of lower water demand as a result of the gypsum being wet when added to the kettle. U.S. Pat. Nos. 4,238,445 to Stone and 4,201,595 to O'Neill both describe processes whereby the plaster is treated with small amounts of liquid water, and ground to give a reduced water demand plaster, although there was some significant degradation of the ability of these plasters to develop strength on setting. In addition, if the plaster from these processes was not used immediately then it needed to be dried to avoid the plaster having unpredictable setting properties. U.S. Pat. No. 4,360,386 to Bounini also describes a process where the plaster is sprayed with an aqueous solution of a solubilizing agent while being ground to give a low water demand plaster. More recently U.S. Patent application Publication 2005/0152827 to Bold describes a multistep process involving treating beta plaster with a water and/or steam at 75 to 99° C., followed by curing and drying. In general the water spray/curing/drying processes result in an increase in residual gypsum content such that the treated plaster contains 3-7% dihydrate.
It is possible to reduce the water demand by these processes in the order of 15-30% but all of these forced aging processes are costly to implement in one form or another. In the case of aridization it is necessary to add soluble salts to the plaster, restricting its use in gypsum board applications and resulting in corrosion problems with the equipment in plaster applications. There are several treatments that are basically different ways of moistening, curing and drying. In general these processes limit production rates and require significant capital investment. In addition, as described recently by Bold, the two main concerns are unintended rehydration, which creates dihydrate, acting as crystallization seeds in plaster slurries as well as build-ups or scaling in the equipment. The formation of dihydrate can result in early stiffening of the setting mix, and yet the aged plaster is slow to dissolve resulting in a long dragged out final set. Overall the setting properties of this type of slurry make it very difficult to use in a rapid production process. As a result of these problems, the post-calcination treatment of beta plaster has had limited application, especially in the production of gypsum board. Aridization is commonly used for industrial plasters but a process to give a low water demand plaster without the addition of soluble salts would be welcomed by the industry.
It would seem that an alpha plaster would be more ideal for many of these beta plaster applications, but the production of alpha hemihydrate is much more costly and difficult to perform. In addition, the properties of an alpha plaster do not lend themselves very easily to processes where the hemihydrate slurry must set very quickly to give a low density, lightweight product, such as gypsum board.
If an application required a hemihydrate plaster intermediate between a typical alpha and beta plaster, the conventional manner to provide this product is to build two production facilities, one for alpha and one for beta, along with a blending plant to allow the production of a plaster intermediate between these two types of materials. U.S. Pat. No. 6,964,704 to Cox describes a process whereby gypsum is briquetted and then calcined in an autoclave to give a material that is intermediate in performance.
One of the ways that the gypsum industry uses to measure the setting properties of a hemihydrate plaster is to measure the temperature rise curve that results from the exothermic hydration of hemihydrate to gypsum. Different companies have different procedures/techniques to monitor this property. It is generally desired in the manufacture of gypsum board for the setting process to start off slowly to allow the paper face liners to be wet by the slurry, but to finish quickly so that the hydration process is as complete as possible before the board enters the dryer. One commonly used technique is to determine the maximum slope of the hydration curve (° C. per minute), with the preferred behavior being a very low slope immediately after mixing, and the maximum slope appearing very late in the overall hydration process. In this case, the hemihydrate board stucco is setting very fast until almost the very end of the hydration time. This is commonly associated with improved strength properties of the final slurry. The beta plasters perform very well by this measure, giving a set curve as exemplified by FIG. 1. A typical alpha plaster however, will have a higher overall temperature rise because of the lower water demand and lower mix heat capacity, but the overall setting process near the end of hydration is very sluggish and takes a long time to finish.
Other gypsum plaster applications require different setting properties. Wall plasters require more strength than would be typically found for a board plaster but require the “body” exhibited by a beta plaster but not by an alpha plaster. Molding plasters require the ability to provide accurate reproductions of detail and good strength properties, along with well-controlled expansion/contraction properties. Set control and crystal habit modifiers can be used to modify the properties of gypsum plasters to fine tune the performance needed, but in general the starting point has been an alpha plaster, a beta plaster, or a blend of the two.
The most commonly used additive to control the hemihydrate setting process is ground gypsum accelerator, effectively to act as seed crystals that provide a larger surface area of gypsum for the dissolved calcium and sulfate ions to crystallize upon. Ground gypsum accelerators are made in many forms by several processes in order to maximize or stabilize the activity of the gypsum crystal surface. Another type of accelerator also exists, commonly called chemical accelerators, which cause the chemical processes of dissolving the hemihydrate and transporting the calcium and sulfate ions to the growing gypsum crystals to take place more quickly. Typical chemical accelerators are potassium and aluminum sulfates, or other soluble sulfates, or sulfuric acid. Chemicals that increase ionic strength or increase the solubility of the hemihydrate more than that of the gypsum are also chemical accelerators.
There are several chemicals that can retard the rate of the hydration process as well. These materials are typically chelating agents that can interfere with the chemical activity of the calcium ions, or chemicals that interfere with the dissolving of the hemihydrate or chemicals that block the surface of the gypsum crystals from receiving soluble calcium and sulfate ions. Typical commercial retarders are diethylene triamine pentaacetic acid (DTPA), citric acid, tartaric acid and hydrolyzed keratin proteins; but many chemical compounds that adsorb on the surface of gypsum crystals will retard the hemihydrate setting process. Sugars found in lignin sulfonates, polyacrylic acids and polyphosphates, for example, are all effective retarders although they may be added to a setting hemihydrate slurry for another reason such as a dispersing agent.